At least one purpose associated with the addition of a stabilizer to a polymeric resin is to prevent deterioration of the polymers derived from the resin during processing at high temperatures and also to permit the manufacture of products with increased intrinsic quality attributable at least in part to increased resistance to thermal and light degradation during their intended use.
Many organic phosphites have been used as stabilizers. Among them are the commercially significant pentaerythritol diphosphites with either a spiro configuration or a caged configuration. These pentaerythritol diphosphites are particularly useful since they are thermally stable, have a high decomposition temperature and are of low volatility. Their high degree of intrinsic stability, especially under conditions of high humidity, is at least partially responsible for their satisfactory performance in inhibiting discoloration of polyolefins, typically caused by high temperature.
Many processes have been proposed for producing bis(alkylphenyl)pentaerythritol phosphites. Phosphites of this type have generally in the past been prepared by one of two methods: (a) the reaction of an alkylphenol with dichloropentaerythritol disphosphite, as shown in U.S. Pat. No. 3,310,609, and (b) sequential transesterification reactions beginning with the reaction of pentaerythritol with an appropriate trialkylphosphite or triphenylphosphite to form pentaerythritol diphosphite which then undergoes the second transesterification reaction with an alcohol or a phenol, to form the desired bis(alkyl) or bis(alkylphenyl)pentaerythritol diphosphite, as illustrated in U.S. Pat. Nos. 4,305,866 and 4,665,211.
Although the aforementioned methods are useful they each suffer from certain disadvantages. The first identified method requires the use of a solvent because of the solid nature of pentaerythritol. The use of a solvent represents an additional cost and it must be removed and recovered, all of which negatively impact the process economics and steps required to synthesize a final product. The second method also has its own disadvantages such as multiple steps, each of which requires purification, which tends to add long reaction cycles, low yield and additional expense.
U.S. Pat. No. 5,103,035 also teaches a method for preparing pentaerythritol diphosphites in a chlorinated solvent and in the presence of a heterocyclic tertiary amine catalyst. Although it is stated that a highly pure product was obtained, the process involves multiple steps, e.g., adding ammonia gas to remove residual hydrogen chloride or other bound acid species, followed by filtering to remove ammonium salts, and crystallizing the product with isopropyl alcohol. These additional steps result in a relatively low yield.
U.S. Pat. No. 5,364,895 also teaches a method for preparing pentaerythritol diphosphites which involves a solvent such as heptane and/or toluene and the reactants 2,4-dicumylphenol and PCl3 and pentaerythritol, optionally with the addition of a trialkanol amine.
U.S. Pat. No. 5,438,086 also teaches a method for preparing pentaerythritol diphosphites which involves a transesterification reaction of triphenylphosphite with pentaerythritol and phenol using a sodium metal catalyst; followed by distillation to remove unwanted by-products followed by the addition of dicumylphenol and a sodium metal catalyst.
Thus, it is apparent that method for effective preparation of bis(alkylphenyl)pentaerythritol diphosphites in a more economical way while achieving high yield and high purity is still to be sought.